I've often wondered if opening windows while AC is running and outside temp is below inside temp is less efficient than keeping them closed.
My thought is I should keep them closed due to extra load on the AC to dehumidify the outside air. Or open windows and turn AC to fan-only mode to prevent stagnant air in rooms without windows.
Unless you have a very strange setup, centralized A/C does not have humidity sensors nor run just to dehumidify. Nor does removing humidity increase the load on the A/C system. Removing humidity is just a happy by-product of how centralized A/C works. So no, you would not be increasing load for that reason.
The above comment is incorrect, even for systems without integrated dehumidifiers. AC systems absolutely do have a higher load with higher humidity because ambient water will condense on the evaporator coils if the dewpoint is higher than the evaporator temperature (which it typically is).
That said, it still may be more efficient to open your windows, depending on the humidity.
I stand corrected. Apparently 60% humidity or so is the line. That said, I live in Florida and haven't ever had an issue, but maybe that's because the A/C runs so much anyway...
FWIW I had a new Lennox heat pump installed a couple years ago, and I can (but don't) configure it to "cool to dehumidify", and it tells me the humidity on the thermostat. I don't know if this is common in new units, or if I'm just special.
Based on some of the home building and HVAC videos I’ve seen on YouTube, it seems pretty common for high-end new home construction in the U.S. (especially the south) to have separate AC and dehumidifier units.
Right. It takes into account both temp and humidity and either can trigger the cooling. It is a two stage system, so usually on the first stage is enough to get the humidity low enough to feel better.
Happy in some times, unhappy in others. There are days I'd love if the A/C would have a humidifier built-in, for those winter months when you might want to use A/C to warm the interior.
A good chunk of the United States (and a number of other countries) uses forced air natural gas heating. If you fall into that group, you can install a "whole-house humidifier" that injects steam into the hot air plenum, distributing it around the house. This makes the air less dry (obviously) but also has the side-effect of actually reducing the need for heating because air holding moisture feels warmer than air without (hence the oppressive heat in the humidity of summer), meaning you can lower the thermostat.
Unfortunately I have only seen commercial humidification units that boil water into steam with the use of natural gas. Without exception, everything for sale for home use uses electricity (you'll need 230V minimum, single phase will do just fine) to boil the water, which is costly (though this year natural gas prices have risen or even doubled, but even in states with cheap electricity it's probably still cheaper to use natural gas).
The moisture can promote mold in the ductwork though. We had our furnace replaced a few years ago and had the central humidifier removed. The interior of the ductwork was black with mold near the humidifier so that was also replaced.
Be careful you’re not running it independently of whether or not the furnace is actually on. 120V generally can’t evaporate enough water in the short window while the furnace is running and many techs will set it up to run at all times (with the blower but not the furnace) to compensate. This lets mildew or mold grow in your ducts.
One of the Daikin models for Japan apartments/condos (known as mansions) has a tankless humidifier; it is able to condensate humidity from outdoor air and introduce it into the home. On a cold winter day it is able to raise the relative humidity from around 30% to about 45% or so. うるるとさらら is the name of their series of things that humidify.
HVAC can do that and it’s not too expensive. You do need to run water to your unit though and there’s quite a bit of maintenance as humidifiers create mold, etc.
Some AC units have two modes - cooling and dehumidifying. When in cooling mode it will blow a lot of air to keep evaporator as hot as possible, so less humidity is condensing on it (but there will still be some). In dehumidifying mode it will blow less air but keep evaporator cool, so more condensation for the same amount of air cooling.
Source - I have Kaisai mini-split at work and I've observed how each mode works (with my hand). There's probably more energy used per cooling amount during dehumidifying, because it's bigger temperature differnce (which means lower COP) PLUS condensing water (water phase change energy is big).
My thought is I should keep them closed due to extra load on the AC to dehumidify the outside air. Or open windows and turn AC to fan-only mode to prevent stagnant air in rooms without windows.